1. Field of the Invention
The present invention relates to technology for coding and transmitting video data through a wireless channel, and more particularly to an apparatus and method for matching compressed video data in a wireless fading environment.
2. Description of the Related Art
When multimedia data including video and audio data is transmitted through a wireless channel, the receiving side may detect data errors in the multimedia data due to a time varying characteristic of the wireless channel. This causes the transmitted video and audio data to be deteriorated. In other words, the wireless channel is frequently subjected to fast fading and slow fading due to a multi-path, an environmental factor, and the movement of a terminal, etc. A conventional multimedia transmission scheme and a codec interface scheme have been proposed for improving the transmission quality in such a wireless environment.
FIG. 1 is a block diagram showing an apparatus for matching compressed video data under a general wireless environment. The apparatus includes an encoder 10 for encoding the video data to output compressed bit stream, a streamer 12 for processing the compressed bit stream output from the encoder 10 to generate a transmission stream, and an MAC/PHY unit 14 for creating MAC frames at a Medium Access Control layer using the transmission stream generated from the streamer 12 and converting the MAC frames into analog signals to transmit the converted MAC frames through a network, i.e. a transmission medium (wireless medium). The apparatus also includes a channel-error measurement unit 16 for measuring Signal-to-Noise Ratio (SNR) or Carrier-to-Noise Ratio (CNR) in the MAC/PHY unit 14 (in the case of FIG. 1, CNR is measured), an error ratio calculation unit 18 for calculating Bit Error Ratio (BER) or Frame Error Ratio (FER) using SNR or CNR measured by the channel-error measurement unit 16, and a temporal Scalable Video Coding (SVC) selector 24 and SNR SVC selector 22 for identifying BER or FER calculated by the error ratio calculation unit 18 and selecting a frame per rate or a bit ratio suitable for transmitting the multimedia, especially the video data, to provide a frame selection signal or a bit ratio selection ratio to the encoder 10.
In FIG. 1, the encoder 10 is configured and operates as a general H.264/Advanced Video Coding (AVC) encoder. The encoder 10 includes a motion estimation unit 110, a motion compensation unit 120, an intra prediction-mode selection unit 170, an intra prediction unit 190, a subtraction unit 130, a Discrete Cosine Transformation (DCT) unit 140, a quantization unit 150, a reordering unit 160, an entropy encoder 165, an inverse quantization unit 155, an inverse DCT transformation unit 145, an addition unit 135, and a filter 180.
The encoder 10 includes two data flow pathways, one of which is a forward pathway proceeding from the left side to the right side and the other which is a reconstruction pathway proceeding from the right side to the left side in FIG. 1.
First, the forward pathway will be described. A current input picture (frame) 101 is provided for prediction-encoding. The frame 101 is processed by units of macro-blocks corresponding to the original image of 16×16 pixels. Each macro-block is encoded in an intra-mode or an inter-mode. In the intra-mode or inter-mode, the prediction macro-block P is created, based on a reconstructed frame.
In the inter-mode, the prediction macro-block P is created in such a manner that the motion estimation unit 110 presumes the motion of one or more reference frames 102, and the motion compensation unit 120 compensates the motion of the reference frames 102. The reference frame 102 may be one or more previous frames that have been already encoded and reconstructed.
In the intra-mode, the prediction macro-block P is formed from samples of the current frame 101, i.e. frame output from the addition unit 135, which is currently encoded, decoded, and reconstructed. The intra prediction unit 190 predicts the block of each macro-block in a preset scan order. The intra prediction-mode selection unit 170 selects the best suitable prediction mode according to the order of scanning the block of the macro-block to be currently encoded from the reconstructed current frame, and the intra prediction unit 190 carries out the intra predictions according to the selected prediction mode.
The subtraction unit 130 creates a difference macro-block between the current macro-block and the prediction macro-block P. The created macro-block is DCT-transformed by the transform unit 140, and then quantized by the quantization unit 150 to create a quantized transformation coefficient X. The output of the quantization unit 150 is provided through the reordering unit 160 to the entropy encoder 165. The compressed bit stream output from the entropy encoder 165 is provided through a Network Abstraction Layer (NAL) to a streamer 12 that transmits or stores the compressed bit stream.
Next, the reconstruction pathway will be described. The quantized transformation coefficient X is inversely quantized by the inverse-quantization unit 155, and then inversely DCT-transformed by the inverse transform unit 145 to create an inverse difference macro-block. This block is not identical with the original difference macro-block due to signal loss, etc.
The prediction macro-block P is merged with the inverse difference macro-block by the addition unit 135, to create the reconstructed macro-block. Such a reconstructed macro-block is a distorted version of the original macro-block. The filter 180 reduces the distortion to create the reconstructed frame 103, which in turn can be used as a criteria frame 102.
The conventional apparatus shown in FIG. 1 first detects the CNR or SNR through the MAC/PHY of a wireless channel in order to transmit multimedia data in a wireless environment. The BER or FER is estimated using the obtained CNR or SNR. If the estimated BER is lacking in transmitting the multimedia data, especially video data, the quality of the video data is degraded to reduce the bit rate or frame per rate. When apparatus is applied to a home network or a car network, it is possible to construct the multimedia apparatus that has reduced sensitivity to the fading change of the wireless channel because the change of the wireless channel is not serious. However, the apparatus shown in FIG. 1 cannot cope with a fast moving portable terminal, i.e. the change of the channel due to the rapid change of the environment.